1. Field of the Invention
This invention relates to hardfacing coatings on a metallic work piece, and more particularly to deposition of a hardfacing coating on roller cone surfaces of a rock bit in selected patterns and thicknesses.
2. Description of the Related Art
Earth boring devices, such as rock bits used in petroleum and rock drilling applications, include surfaces exposed to erosive wear due to contact with geological formations. Two types of rock bits are commonly used: tungsten carbide insert rock bits and milled tooth rock bits.
Tungsten carbide insert rock bits are generally used to drill hard formations because of the enhanced ability of tungsten carbide inserts to penetrate hard formations. However, the tungsten carbide inserts are mounted in a relatively soft metal, such as steel, that forms the body of the cutter cone. The relatively soft metal cutter body which holds the inserts in place may be abraded or eroded away when subjected to a high abrasive drilling environment. This abrasion or erosion occurs primarily due to the presence of cuttings from the formation, the direct blasting effect of the drilling fluid utilized in the drilling process, and the rolling and sliding contact of the cone body or cone shell with the formation. When the material supporting the inserts is eroded or abraded away to a substantial extent, the drilling forces being exerted on the inserts may either break the inserts or force them out of the cutter cone when they engage the formation. As a result, the bit may no longer be effective in cutting the formation. Moreover, the loose inserts that break off from the cutter cone may damage other inserts and the cutter cone, and eventually may lead to failure of the cutter cone.
When drilling relatively soft but abrasive formations, individual cutting inserts may penetrate entirely into the abrasive formation, causing the formation to come into contact with the cutter cone. When this contact occurs, the relatively soft cone shell material will erode away, namely at the edges of the surface lands, until the previously embedded portion of the insert becomes exposed and the retention ability in the cone shell is reduced, which may result in the loss of the insert and reduction of the life of the bit. To protect the cutter cone from erosion, hardfacing material, such as tungsten carbide, has been applied to the cone surfaces by a variety of well known methods.
Milled tooth rock bits are another important type of rock bits used in petroleum and mining drilling applications. A milled tooth bit has a roller cone with teeth protruding from the surface of the cone for engaging the rock. The teeth are made of hardened steel and generally are triangular in a cross-section (as observed in a plane perpendicular to the axis of the cone). The principal faces of the milled teeth that engage the rock usually are dressed with a layer of hardfacing material to increase wear-resistance.
Different methods have been developed, with varying degree of success, for applying hardface coatings to various wear prone surfaces on rock bits. For example, to prevent erosion, small, flat-top compacts made of hard material may be placed in vulnerable cone areas using a silicate bonding agent. Wear resistant material can be applied to exposed surfaces of a cutter cone by thermal spraying, plasma arc welding, or other conventional welding.
Milled tooth rock bits present a particular problem in applying hardfacing material in such a way as to most effectively lengthen the overall useful life of the bit. For example, various hardface material compositions and particle size distributions are disclosed in U.S. Pat. No. 4,836,307 issued to Keshavan et al, U.S. Pat. No. 5,051,112 issued to Keshavan et al, and U.S. Pat. No. 5,492,186 issued to Overstreet et al which discloses a bimetallic hardface gauge facing. Others have disclosed, for example, applying an additional layer of hardfacing on one flank of the milled teeth to improve the wear resistance of the teeth. See for example, U.S. Pat. No. 5,791,423 issued to Overstreet et al. An additional layer of hardfacing is typically applied by passing the teeth a second time through the apparatus used to apply the hardfacing. Still others have disclosed various shapes for the teeth formed so that when a relatively uniform thickness layer of hardfacing material is applied thereto, the result is improved wear resistance. See for example, U.S. Pat. No. 5,445,231 issued to Scott et al wherein a tooth substrate is shaped so that a hardface applied therein which has a uniform external surface provides hardface having greater thickness on one tooth flank than on the other. U.S. Pat. No. 5,351,771 issued to Zahradnik shows a bit tooth crest having indentations where the crest contacts the edges of the flanks to provide thicker hardface in such locations when hardface is applied so as to have a uniform external surface. U.S. Pat. No. 5,351,769 issued to Scott et al shows the crests of the teeth milled to have a recess on one side thereof for application of hardface material.
FIG. 1, for example, shows one type of prior art hardface application to the teeth on a drill bit. The teeth, shown generally at 10 include thereon a hardface coating 12 upon a tooth substrate 14. The hardface 12 has a first, substantially constant thickness t1 along one flank, such as the trailing edge flank, and a different substantially constant thickness t2 along the other flank of each substrate 14. As previously explained, the greater thickness can be obtained by a second pass through the application system (such as welding). In any event, the teeth shown in FIG. 1 have substantially constant thickness of hardfacing on each flank thereof.
FIG. 2 illustrates typical manual application of hardfacing using a manual welding apparatus such as a torch 38 and welding rods 39. The heat 37 from the torch 38 melts the rods 39 whereupon the material is fusibly bonded to the substrate 25. There is no provision in this technique for selecting different hardfacing thickness between the flanks 27, 29 crest 31 and sides 33, 35 of the substrate 25.
Illustrations of specialized shapes for the substrates are shown in FIGS. 3 and 4, wherein a substantially uniform thickness hardfacing is applied to the shapes shown in FIGS. 3 and 4 to result in bit teeth having selected wearing characteristics. The specialized shapes such as shown at 237 in FIG. 4 and 137 in FIG. 3 can have the undesired side effect of creating stress risers in the substrates 225 and 125 respectively.
The hardfacing application techniques known in the art can provide improved resistance to wear of the teeth but may have the unintended side effect of reducing bit performance by reducing debris clearance between adjacent teeth, as well as increasing residual stresses in the teeth where specialized tooth structures are used.
One aspect of the invention is a method for applying hardfacing to teeth on a drill bit. The method includes applying the hardfacing to the teeth so as to have a thickness selected to correspond to the position along the surface of the tooth.
In one embodiment of the method of the invention, the hardfacing thickness is increased near the crest of at least one tooth, and is decreased near the flank root of at least one flank of that at least one tooth.
In another embodiment of the method, the hardfacing is applied so as to have a thickness which results in the formation of a self-sharpening cap proximal to the crest of at least one tooth.
In another embodiment of the method, an angle subtended by flanks of the tooth is selected to provide increased strength to the tooth, and the hardfacing is applied to the tooth to cause exterior surfaces of the hardfacing corresponding to the flanks to subtend an angle selected to provide increased penetration of the tooth through earth formations while drilling.
In another embodiment of the method, the hardfacing is applied to a greater thickness on a leading edge of the tooth than on the trailing edge of the tooth.
Another aspect of the invention is a drill bit cutting element. The cutting element includes a substrate, and a hardface overlay applied to an exterior of the substrate so that a thickness of the hardface overlay is selected to correspond to a position about the exterior surface of the substrate.
In one embodiment of the cutting element, the hardfacing thickness is increased near the crest of the cutting element, and is decreased near the flank root of at least one flank of the cutting element.
In another embodiment of the cutting element, the hardfacing is applied so as to have a thickness which results in a self-sharpening cap proximal to the crest of the cutting element.
In another embodiment of the cutting element, an angle subtended by flanks of the cutting element substrate is selected to provide increased strength to the cutting element, and the hardfacing is applied to the substrate to cause exterior surfaces of the hardfacing corresponding to the flanks to subtend an angle selected to provide increased penetration of the cutting elements through earth formations while drilling.
In another embodiment of the cutting element, the hardfacing is applied to a greater thickness on a leading edge of the cutting element than on a trailing edge of the cutting element.